Thermal inkjet print cartridges operate by rapidly heating a small volume of ink to cause the ink to vaporize and be ejected through one of a plurality of orifices so as to print a dot of ink on a recording medium, such as a sheet of paper. Typically, the orifices are arranged in one or more linear arrays in a nozzle member. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved relative to the paper. The paper is typically shifted each time the printhead has moved across the paper. The thermal inkjet printer is fast and quiet, as only the ink strikes the paper. These printers produce high quality printing and can be made both compact and affordable.
An inkjet printhead generally includes: (1) ink channels to supply ink from an ink reservoir to each vaporization chamber proximate to an orifice; (2) a metal orifice plate or nozzle member in which the orifices are formed in the required pattern; and (3) a silicon substrate containing a series of thin film resistors, one resistor per vaporization chamber.
To print a single dot of ink, an electrical current from an external power supply is passed through a selected thin film resistor. The resistor is then heated, in turn superheating a thin layer of the adjacent ink within a vaporization chamber, causing explosive vaporization, and, consequently, causing a droplet of ink to be ejected through an associated orifice onto the paper.
In an inkjet printhead, described in U.S. Pat. No. 4,683,481 to Johnson, entitled "Thermal Ink Jet Common-Slotted Ink Feed Printhead," ink is fed from an ink reservoir to the various vaporization chambers through an elongated hole formed in the substrate. The ink then flows to a manifold area, formed in a barrier layer between the substrate and a nozzle member, then into a plurality of ink channels, and finally into the various vaporization chambers. This design may be classified as a "center" feed design, whereby ink is fed to the vaporization chambers from a central location then distributed outward into the vaporization chambers. Some disadvantages of this type of ink feed design are that manufacturing time is required to make the hole in the substrate, and the required substrate area is increased by at least the area of the hole. Also, once the hole is formed, the substrate is relatively fragile, making handling more difficult. Further, the manifold inherently provides some restriction of ink flow to the vaporization chambers such that the energization of heater elements within a vaporization chamber may affect the flow of ink into a nearby vaporization chamber, thus producing crosstalk which affects the amount of ink emitted by an orifice upon energization of a nearby heater element. More importantly, prior printhead design limited the ability of printheads to have the high nozzle densities and the high operating frequencies and firing rates required for increased resolution and throughput. Print resolution depends on the density of ink-ejecting orifices and heating resistors formed on the cartridge printhead substrate. Modern circuit fabrication techniques allow the placement of substantial numbers of resistors on a single printhead substrate. However, the number of resistors applied to the substrate is limited by the conductive components used to electrically connect the cartridge to external driver circuitry in the printer unit. Specifically, an increasingly large number of resistors requires a correspondingly large number of interconnection pads, leads, and the like. This increase in components and interconnects causes greater manufacturing/production costs, and increases the probability that defects will occur during the manufacturing process. In order to solve this problem, thermal inkjet printheads have been developed which incorporate pulse driver circuitry directly on the printhead substrate with the resistors. The incorporation of driver circuitry on the printhead substrate in this manner reduces the number of interconnect components needed to electrically connect the cartridge to the printer unit. This results in an improved degree of production and operating efficiency. This development is described in U.S. Pat. Nos. 4,719,477 and 5,122,812 which are herein incorporated by reference.
To produce high-efficiency, integrated printing systems as described above, significant research has been conducted in order to develop improved transistor structures and methods for integrating the same into thermal inkjet printing units. The integration of driver components and printing resistors onto a common substrate results in a need for specialized, multi-layer connective circuitry so that the driver transistors can communicate with the resistors and other portions of the printing system. Typically, this connective circuitry involves a plurality of separate conductive layers, each being formed using conventional circuit fabrication techniques.
To create the resistors, an electrically conducting layer is positioned on selected portions of the layer of resistive material in order to form covered sections of the resistive materials and uncovered sections thereof. The uncovered sections ultimately function as heating resistors in the printhead. The covered sections are used to form continuous conductive links between the electrical contact regions of the transistors and other components in the printing system. Thus, the layer of resistive material performs dual functions: as heating resistors in the system, and as direct conductive pathways to the drive transistors. This substantially eliminates the need to use multiple layers for carrying out these functions alone.
A selected portion of protective material is then applied to the covered and uncovered sections of resistive material. Thereafter, an orifice plate having a plurality of openings through the plate was positioned on the protective material. Beneath the openings, a section of the protective material which was removed forms ink firing cavities or vaporization chambers. Positioned at the bottom surface of each chamber is one of the heater resistors. The electrical activation of each resistor causes the resistor to rapidly heat and vaporize a portion of the ink in the cavity. The rapidly formed (nucleated) ink bubble ejects a droplet of ink from the orifice associated with the activated resistor and ink firing vaporization chamber.
To increase resolution and print quality, the printhead nozzles must be placed closer together. This requires that both heater resistors and the associated orifices be placed closer together. To increase printer throughput, the width of the printing swath must be increased by placing more nozzles on the print head. However, adding resistors and nozzles requires adding associated power and control interconnections. These interconnections are conventionally flexible wires or equivalent conductors that electrically connect the transistor drivers on the printhead to printhead interface circuitry in the printer. They may be contained in a ribbon cable that connects on one end to control circuitry within the printer and on the other end to driver circuitry on the printhead. An increased number of heater resistors spaced closer together also creates a greater likelihood of crosstalk and increased difficulty in supplying ink to each vaporization chamber quickly.
Interconnections are a major source of cost in printer design, and adding them in increase the number of heater resistors increases the cost and reduces the reliability of the printer. Thus, as the number of drivers on a printhead has increased over the years, there have been attempts to reduce the number of interconnections per driver. A matrix approach offers an improvement over the direct drive approach, yet as previously realized a matrix approach has its drawbacks. The number of interconnections with a simple matrix is still large and still results in an undesirable increase in the number of interconnections.
Another concern with inkjet printing is the sufficiency of ink flow to the paper or other print media. Print quality is also a function of ink flow through the printhead. Too little ink on the paper or other media to be printed upon produces faded and hard-to-read printed documents. Ink flow from its storage space to the ink firing chamber has suffered, in previous printhead designs, from an inability to be rapidly supplied to the firing chambers. The manifold from the ink source inherently provides some restriction on ink flow to the firing chambers thereby reducing the speed of printhead operation as well as resulting in crosstalk.
To resolve these needs of increased printing speed, resolution and quality, increased throughput, reduced number of interconnections, and improved ink flow control for higher frequency firing rates, a modem design of ink jet printer printheads is desirable.